Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Helical compression spring for an actuator for opening and closing a door or a
tailgate of a
car
Description
Technical Field
[1] The invention relates to a helical compression spring for an actuator
for opening and
closing a door or a tailgate of a car. The invention further relates to an
actuator for
opening and closing a door or a tailgate of a car.
Background Art
[2] SUVs (Sports Utility Vehicles) know increased popularity. SUVs have a
large ¨ and thus
heavy - tailgate. It is known to use helical compression steel springs in the
actuators to
open and close the tailgate of such SUVs.
[3] There is an increased tendency to use motor operated tailgate opening
and closing
actuators. US2018/0216391A1 and US2017/0114580A1 disclose such actuators.
[4] In such typical actuators, the tailgate is opened by releasing the
forces of a helical steel
wire spring operating in compression mode. The tailgate is closed by the
operation of a
motor; whereby the motor compresses the helical steel wire spring. The helical
steel wire
spring for such applications has to meet very stringent requirements.
According to a first
requirement, the helical spring must have a small diameter, in order to make
the tailgate
opening and closing system as compact as possible. The spring must be able to
withstand the high compressive forces in a consistent way. Relaxation of the
spring must
be low, as spring relaxation modifies the spring forces for a given
compression, which
would be negative for the operation of the tailgate opening and closing
actuator.
Furthermore, the spring must have sufficient fatigue resistance, in that it
must survive the
required number of opening and closing cycles of the tailgate at high load of
the spring.
Because of the size of the tailgates of SUVs, the springs used have a long
length.
[5] It is known to use steel wires having a martensitic microstructure for
producing helical
springs for tailgate opening and closing actuators of cars. Steel wires having
a martensitic
microstructure are typically manufactured by hardening and tempering heat
treatment
operations.
[6] DE202004015535U1 describes a tailgate opening and closing system of a
car. The
system comprises a helical steel wire spring. The spring is made from a steel
wire having
a diameter of at least 1 mm. The steel alloy out of which the steel wire is
made comprises
0.5 ¨ 0.9 `)/0 by weight of carbon, 1 ¨2.5 cYo by weight of silicon, 0.3 ¨ 1.5
cYo manganese,
0.5 ¨ 1.5 cYo by weight of chromium, iron and impurities. The steel alloy
optionally
comprises 0.05 ¨ 0.3 cYo by weight of vanadium and/or 0.5 ¨ 0.3 cYo by weight
of niobium
and/or tantalum. The steel wire is made via a patenting operation followed by
wire
drawing. The steel wire is then hardened and tempered to obtain a martensitic
microstructure, a tensile strength higher than 2300 N/mm2 and a reduction of
the cross
sectional area at break of more than 40%. The obtained steel wire is cold
formed into a
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helical spring, which is then stress relieved at a temperature between 200 C
and 400 C.
The spring can be shot peened to increase its durability.
[7] Helical springs exist that are made with hard drawn steel wire.
European Standard EN
10270-1:2011 is entitled "Steel wire for mechanical springs ¨ Part 1: Patented
cold drawn
unalloyed spring steel wire". Although the title refers to unalloyed spring
steel wire,
section 6.1.2 of the standard indicates that the addition of micro-alloying
elements may
be agreed between the manufacturer and the purchaser. The standard
differentiates steel
spring wire in two ways. The first way is according to static duty (S) or
dynamic duty (D).
The second way is according to tensile strength, low (L), medium (M) or high
(H). The two
ways combined provide 5 grades of spring steel wire (SL, SM, DM, SH and DH)
for which
the mechanical properties (among which the tensile strength Rm) and quality
requirements are given in Table 3 of standard EN 10270-1:2011 as a function of
the steel
wire diameter. As an example, for steel wire of diameter between 3.8 and 4 mm,
the
tensile strength Rm for grade DH (the grade which has the highest specified
tensile
strength) needs to be between 1740 and 1930 MPa.
Disclosure of Invention
[8] The first aspect of the invention is a helical compression spring,
preferably for use in an
actuator for opening and closing a door or a tailgate of a car. The helical
compression
spring has an outer diameter between 15 and 50 mm. The helical compression
spring
comprises a helically coiled steel wire. The diameter d (in mm) of the steel
wire is
between 2 and 5 mm. The steel wire comprises a steel alloy, consisting out of
between
0.8 and 0.95 wt% C; between 0.2 and 0.9 wt% Mn; between 0.1 and 1.4 wt% Si;
between
0.15 and 0.4 wt% Cr; optionally between 0.04 and 0.2 wt% V; optionally between
0.0005
and 0.008 wt% B; optionally between 0.02 and 0.06 wt% Al; unavoidable
impurities; and
the balance being iron. The steel alloy has a carbon equivalent higher than 1.
The carbon
equivalent is defined as: C wt% + (Mn wt% /6) + (Si wt% / 5) + (Cr wt% / 5) +
(V wt% / 5).
The microstructure of the steel wire in the helical compression spring is
drawn lamellar
pearlite.
[9] Surprisingly, the helical compression spring of the invention is
ideally suited for use in an
actuator for opening and closing a door or a tailgate of a car. Martensitic
steel wires are
described in the prior art for use in helical compression springs for
actuators for opening
and closing tailgates. The steel wires required for helical springs for
actuators for opening
and closing tailgates must have a diameter between 2 and 5 mm and must have a
high
strength and sufficient ductility. Hardened and tempered steel wires (which
have a
martensitic microstructure) of these diameters have highest strength.
Helically coiled
springs made with such hardened and tempered steel wires and having been
subjected
to the standard post treatments (e.g. stress relieving and shot peening)
provide the
combination of excellent fatigue life and low relaxation of the spring force.
Because of the
high demands for springs for actuators for opening and closing tailgates (high
compressive forces, low relaxation allowed and fatigue life requirements)
which match
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perfectly with the known properties of martensitic steel wires, the skilled
person has a
technical prejudice to use martensitic steel wires and not to use hard drawn
wires (hard
drawn wires have a drawn pearlitic microstructure) for the production of
helical
compression springs for actuators for opening and closing tailgates. The steel
alloys
selected in the invention surprisingly provide steel wires with drawn
pearlitic
microstructure which have the combination of steel wire properties (strength,
yield
strength, ductility) required to obtain helical compression springs that
satisfy the
demanding requirements for use in actuators for opening and closing a door or
a tailgate
of a car.
[10] Preferably the carbon content of the steel alloy is less than 0.93
wt%, more preferably
less than 0.9 wt%.
[11] Preferably, the steel alloy comprises less than 0.35 wt% Cr, more
preferably less than 0.3
wt% Cr.
[12] When the steel alloy comprises V, preferably the steel alloy comprises
less than 0.15
Wt% V.
[13] Preferably, the steel alloy comprises between 0.02 and 0.06 wt% Al. It
is a benefit of such
embodiment that better helical compression springs can be obtained thanks to
the higher
ductility of the steel wire used to manufacture the helical compression
spring.
[14] Preferably, the helical compression spring has an outer diameter less
than 40 mm.
[15] Preferably, the helical compression spring has a length in unloaded
condition of more
than 40 cm. More preferably of more than 60 cm.
[16] Preferably the length of the spring in unloaded condition is less than
1000 mm.
[17] Preferably, the helical compression spring has a spring index between
3 and 8. The
spring index is the ratio of the diameter of the spring (wherein the diameter
of the spring
for calculating the spring index is the average between the outer diameter and
the inner
diameter of the spring in unloaded condition) over the diameter of the steel
wire of the
spring.
[18] Preferably, the steel alloy has a carbon equivalent higher than 1.05;
more preferably
higher than 1.1.
[19] Preferably, the steel alloy has a carbon equivalent below 1.4, more
preferably below 1.3.
[20] Preferably, the diameter of the steel wire is between 2 and 4 mm, more
preferably
between 2.5 and 3.8 mm.
[21] Preferably, the helical compression spring has a pitch angle between 5
and 10 . Such
helical compression springs can be beneficially used in tailgate opening and
closing
actuators of cars with trunk closing systems.
[22] Preferably, the steel wire used for helically coiling the helical
compression spring has a
tensile strength Rm (in MPa) higher than the value calculated by the formula
2680 ¨
390.71 * In(d). More preferably, the tensile strength Rm (in MPa) of the steel
wire is higher
than the value calculated by the formula 2720 ¨ 390.71 * In(d); more
preferably higher
than the value calculated by the formula 2770 ¨ 390.71 * In(d); and even more
preferably
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higher than the value calculated by the formula 2800 ¨ 390.71 *In(d). With the
function
"In(d)" is meant the natural logarithm of the diameter d (in mm) of the steel
wire. The
tensile test to measure the mechanical properties of the steel wires is
conducted
according to ISO 6892-1:2009 entitled "Metallic materials -- Tensile testing --
Part 1:
Method of test at room temperature".
[23] Preferably, the percentage reduction of area Z at break in tensile
testing of the steel wire
used for the production of the helical compression spring is more than 40%.
The
percentage reduction of area Z is calculated as: Z=100*(So-S,)/So, So being
the original
cross section of the steel wire and Su being the smallest cross section of the
steel wire
after fracture in tensile testing.
[24] Preferably, the steel alloy comprises between 0.3 and 0.6 wt% Mn; or
the steel alloy
comprises between 0.6 and 0.9 wt% Mn.
[25] Preferably the steel wires comprises in the spring at least 95% - and
more preferably at
least 97% - by volume of drawn lamellar pearlite.
[26] In a preferred embodiment, the volume percentage of bainite in the
microstructure of the
steel wire is between 0.2% and 2%, more preferably below 0.5%. Such
embodiments
have surprisingly shown to be particularly beneficial for the invention. When
the
microstructure comprises such amounts of bainite, it is an indication that the
lamellar
pearlite is very fine, favorable to achieve optimum spring formation and
excellent
mechanical spring properties, without the bainite creating negative effects.
The limited
amount of bainite is important for the ductility of the steel wire. The low
amount of bainite
can be achieved by a proper patenting operation in the production process of
the steel
wire. The volume percentage bainite in the microstructure of the wire can be
determined
via optical microscopy or scanning electron microscopy using an appropriate
etchant.
[27] Optionally, a phosphate coating can be applied on the steel wire
before the wire drawing
process. The step of helically coiling the steel wire into the helical
compression spring
can then be performed with the steel wire comprising the phosphate coating at
its
surface. Such embodiment provides a better helical compression spring because
the
phosphate coating facilitates the wire drawing and spring coiling operation.
Therefore, in
a preferred embodiment, the helically coiled steel wire comprises a phosphate
coating. In
a more preferred embodiment, a thermoset coating layer or a coating layer
comprising
zinc and/or aluminum flakes in a binder (preferably an inorganic binder is
used) is applied
onto the phosphate coating layer. In an even more preferred embodiment, the
helically
coiled steel wire is provided with a layer of flock. With flock is meant a
layer of short
textile fibers, e.g. polyamide fibers, bonded by means of an adhesive onto the
helical
compression spring.
[28] In a preferred embodiment, the helically coiled steel wire
comprises a metallic coating
layer. The metallic coating layer comprises ¨ and preferably consists out of -
at least 84%
by mass of zinc; optionally aluminum, optionally 0.2¨ 1 wt% magnesium, and
optionally
up to 0.6 wt% silicon. Such embodiments have the benefit that spring
manufacturing is
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facilitated. Furthermore, no additional (or post-) coating needs to be applied
on the helical
compression spring to provide the spring with corrosion resistance properties.
Furthermore, such metallic coatings avoid the need to apply a flock layer on
the helically
coiled compression spring. Such flock layer has the function of avoiding noise
generation
5 in the actuator for opening and closing the tailgate or door of a car
when driving the car.
The metallic coating of the helically coiled steel wire also prevents the
occurrence of such
noise. Preferably, the metallic coating layer is more than 40 g/m2, more
preferably more
than 80 g/m2. More preferably, less than 120 g/m2. The mass of the metallic
coating layer
is expressed per unit of surface area of the steel wire.
[29] It is known to apply on the already coiled helical compression springs
polymer coatings to
provide the spring with corrosion resistance. Such approach is done on prior
art helical
compression springs made from steel wire having a martensitic microstructure.
Such
coatings can e.g. be thermoset polymer coatings (e.g. comprising an epoxy
backbone or
an acrylic backbone or an combined epoxy/acrylic backbone) or coatings
comprising zinc
flakes in a binder. The application ¨ according to the invention - of a
metallic coating layer
on the steel wire before or in between drawing operations, and coiling the
helical
compression spring with such steel wire allows eliminating the step of coating
the spring
using thermoset polymer coatings or coatings comprising zinc or aluminium
flakes in a
binder.
[30] In embodiments wherein the helically coiled steel wire comprises a
metallic coating layer,
preferably, the metallic coating layer provides the surface of the helical
compression
spring.
[31] Optionally, when the helically coiled steel wire comprises a metallic
coating layer, the
metallic coating layer comprises other active elements, each in individual
quantities of
less than 1 `)/0 by weight.
[32] Preferably, when the helically coiled steel wire comprises a metallic
coating layer, the
metallic coating comprises at least 88 wt% of zinc, more preferably at least
90 wt% of
zinc. More preferably, the metallic coating layer comprises at least 93 wt% of
zinc.
[33] Preferably, when the helically coiled steel wire comprises a metallic
coating layer, the
metallic coating comprises ¨ and preferably consists out of¨ zinc, at least 4%
by weight
of aluminum - and preferably less than 14 cYo by weight of aluminum -;
optionally between
0.2 and 2 wt cYo magnesium (and preferably less than 0.8 wt% Mg); optionally
up to 0.6
wt% silicon; optionally up to 0.1 wt% rare earth elements, and unavoidable
impurities.
[34] Preferably, when the helically coiled steel wire comprises a metallic
coating layer, the
metallic coating layer comprises - and preferably consists out of - between 86
and 92
wt% Zn and between 14 and 8 wt cYo Al; and unavoidable impurities. Preferably,
such
metallic coating layer has a mass between 35 and 60 g/m2.
[35] Preferably, when the helically coiled steel wire comprises a metallic
coating layer, the
metallic coating layer consists out of zinc, between 3 and 8 wt% aluminum;
optionally 0.2
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¨ 1 wt% magnesium; optionally up to 0.1 wt% rare earth elements; and
unavoidable
impurities. Preferably, such metallic coating layer has a mass between 60 and
120 g/m2.
[36] Preferably, when the helically coiled steel wire comprises a
metallic coating layer, the
metallic coating layer consists out of zinc, between 3 and 8 wt% aluminum;
between 0.2 -
2 wt (and preferably less than 0.8 wt%) Mg; and unavoidable impurities. It is
a particular
benefit that such metallic coating layer can be made thin while still having
good corrosion
protection properties. A thinner coating layer also facilitates coiling of the
helical
compression spring. Such metallic coating layer can e.g. be less than 60 g/m2.
Preferably
between 25 and 60 g/m2.
[37] Preferably, when the helically coiled steel wire comprises a metallic
coating layer, the
mass of the metallic coating layer is between less than 120 g/m2, more
preferably the
mass of the metallic coating layer is between 20 and 80 g/m2, more preferably
less than
60 g/m2 of the surface of the helical compression spring, even more preferably
less than
40 g/m2 of the surface of the helical compression spring.
[38] Preferably, when the helically coiled steel wire comprises a metallic
coating layer, the
metallic coating layer comprises a globularized aluminum rich phase. Such
globularized
aluminum rich phase is created in drawing as the steel wire is heated by the
drawing
energy; and even to a larger extent when a stress relieving heat treatment is
performed
on the helical compression spring after coiling it. It is believed that the
globularized
aluminum rich phase improves the corrosion resistance of the metallic coating
layer; such
that a thinner metallic coating layer can be used.
[39] Preferably, when the helically coiled steel wire comprises a metallic
coating layer, the
coated steel wire comprises an intermetallic coating layer provided between
the steel wire
and the metallic coating layer. The intermetallic coating layer comprises an
Fe,Aly phase.
More preferably the intermetallic coating layer provides between 30 and 65% of
the
combined thickness of the intermetallic coating layer and the metallic coating
layer. The
intermetallic layer is beneficial as it creates the required adhesion of the
metallic coating
layer, in order to allow the steel wire to be coiled into a helical
compression spring without
damage to the metallic coating layer. A thinner intermetallic coating layer
risks to provide
flaking when coiling the spring; a thicker coating risks that coilability is
not good. The
intermetallic coating layer comprising an Fe,Aly phase is obtained when using
a double
dip process to apply the metallic coating layer. A first dip is performed in a
zinc bath. A
Fe,Zny layer is formed on the steel surface. The second dip is performed in a
bath
comprising Zn and Al. In the second bath, the Fe,Zny layer formed in the first
bath is
converted to an intermetallic coating layer comprising an Fe,Aly phase.
[40] Preferably, when the helically coiled steel wire comprises a metallic
coating layer, the
coated steel wire comprises an inhibition layer. The inhibition layer is
provided between
the steel wire and the metallic coating layer. The inhibition layer is
provided by an Fe,Aly
phase. Preferably, the inhibition layer is less than 1 pm thick. A coated
steel wire with
such inhibition layer can be obtained by using a single dip process to apply
the metallic
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coating layer. The steel surface is activated, e.g. via the Sendzimir process,
and the steel
wire is immersed in a bath comprising molten Zn and Al. The steel wire is
wiped after
immersion in the bath and cooled.
[41] In a preferred embodiment wherein the helically coiled steel wire
comprises a metallic
coating layer, the metallic coating layer consists out of zinc and unavoidable
impurities.
More preferably, the mass of such metallic coating layer is more than 80 g/m2,
more
preferably more than 100 g/m2.
[42] In preferred embodiments, the steel alloy comprises between 0.15 and
0.35 wt% Si, or
the steel alloy comprises between 0.6 and 0.8 wt% Si, or the steel alloy
comprises
between 0.8 and 1.4 wt% Si.
[43] In more preferred embodiments wherein the helically coiled steel wire
comprises a
metallic coating, the steel alloy comprises between 0.6 and 1.4 wt% Si; more
preferably
between 0.8 and 1.4 wt% Si. Such embodiments are particularly beneficial, as a
coated
steel wire with high strength can be obtained, as the high amount of Si
prevents loss of
strength of the steel wire in the hot dip process when applying the metallic
coating in an
intermediate step in the wire drawing process.
[44] In a preferred helical compression spring, the steel alloy consists
out of between 0.83 and
0.89 wt% C, between 0.55 and 0.7 wt% Mn, between 0.1 and 0.4 wt% Si, between
0.15
and 0.3 wt% Cr, between 0.04 and 0.08 wt% V, optionally between 0.02 and 0.06
wt% Al;
and unavoidable impurities and the balance being iron.
[45] In a preferred helical compression spring, the steel alloy consists
out of between 0.83 and
0.89 wt% C, between 0.55 and 0.7 wt% Mn, between 0.55 and 0.85 wt% Si, between
0.15 and 0.3 wt% Cr, between 0.04 and 0.08 wt% V, optionally between 0.2 and
0.06
wt% Al; and unavoidable impurities and the balance being iron.
[46] In a preferred helical compression spring, the steel alloy consists
out of between 0.9 and
0.95 wt% C, between 0.2 and 0.5 wt% Mn, between 1.1 and 1.3 wt% Si, between
0.15
and 0.3 wt% Cr; and unavoidable impurities and the balance being iron.
[47] In a preferred embodiment, the helically coiled steel wire has a non-
circular cross section,
preferably a rectangular or square cross section. For embodiments wherein the
cross
section of the helically coiled steel wire is non-circular, the diameter of
this steel wire is
the equivalent diameter. The equivalent diameter is the diameter of a wire
with circular
cross section which has the same cross sectional area as the wire with non-
circular cross
section.
[48] Preferably, the steel wire in the helical compression spring has a
drawing reduction of
more than 75%. The wire rod from which the steel wire has been drawn, or the
steel wire
itself has undergone a patenting operation to create a pearlitic
microstructure; followed by
steel wire drawing operations. The drawing reduction (in %) is defined as 100*
(A0 -
Ai)/A0, wherein Ao equals the area of the cross section of the wire rod or the
steel wire
after patenting and before drawing; and Ai the area of the cross section of
the drawn
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steel wire used to manufacture the spring. During the drawing deformation the
pearlite
grains will be oriented into longitudinal direction of the steel wire. The
level of orientation
of the pearlite grains is determined by the drawing reduction of the steel
wire. The
drawing reduction can be assessed from the evaluation of the drawn lamellar
pearlite
microstructure of the steel wire in the helical compression spring, e.g. by
means of light
optical microscopy on a longitudinal section (i.e. along the longitudinal
direction of the
steel wire in the helical compression spring).
[49] In a preferred helical compression spring, after 20000 compressive
load cycles of the
helical spring between 63% and 37% of its length in unloaded condition, the
load loss at
63% of its length is less than 5% (and preferably less than 3%) compared to
the load at
63% of its length at the first cycle.
[50] The second aspect of the invention is a method for making a helical
compression spring
as in any embodiment of the first aspect of the invention. The method
comprises the
steps of
- providing a steel wire rod, preferably with diameter between 7 and 14 mm;
- patenting the steel wire rod or a steel wire drawn from the steel wire rod,
in order to
obtain a pearlitic microstructure;
- drawing, with drawing reduction more than 75%, the patented steel wire rod
having a
pearlitic microstructure or the patented steel wire having a pearlitic
microstructure;
thereby obtaining a steel wire with drawn pearlitic microstructure, with
diameter d (in mm)
between 2 and 5 mm;
- helically coiling the steel wire into a helical spring;
- optionally performing a thermal stress relieving on the helical spring.
The steel wire rod comprises a steel alloy consisting out of between 0.8 and
0.95 wt% C
(and preferably less than 0.93 wt% C, more preferably less than 0.9 wt% C);
between 0.2
and 0.9 wt% Mn; between 0.1 and 1.4 wt% Si; between 0.15 and 0.4 wt% Cr (and
preferably less than 0.35 wt% Cr, more preferably less than 0.3 wt% Cr);
optionally
between 0.04 and 0.2 wt% V (and preferably less than 0.15 wt% V); optionally
between
0.0005 and 0.008 wt% B; optionally between 0.02 and 0.06 wt% Al; unavoidable
impurities; and the balance being iron. The steel alloy has a carbon
equivalent higher
than 1. The carbon equivalent is defined as: C wt% + (Mn wt% / 6) + (Si wt% /
5) + (Cr
wt% / 5) + (V wt% /5).
[51] In a preferred method, the drawing operation results in a steel wire
with diameter d (in
mm) between 2 and 5 mm and having tensile strength Rm (in MPa) higher than the
value
calculated by the formula: 2680 ¨ 390.71 * In(d). More preferably, the drawing
results in a
steel wire with tensile strength Rm (in MPa) higher than the value calculated
by the
formula 2720 ¨ 390.71 * In(d); more preferably higher than the value
calculated by the
formula 2770 ¨ 390.71 * In(d); and even more preferably higher than the value
calculated
by the formula 2800 ¨ 390.71 *In(d). With the function "In(d)" is meant the
natural
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logarithm of the diameter d (in mm) of the steel wire. The tensile test to
measure the
mechanical properties of the steel wires is conducted according to ISO 6892-
1:2009
entitled "Metallic materials -- Tensile testing -- Part 1: Method of test at
room
temperature".
[52] The patenting step to obtain a pearlitic microstructure can be
performed on the wire rod
or on a steel wire drawn from the wire rod. The patenting step can be
performed as an
inline step in the wire rod production process, e.g. via direct in-line
patenting. The
patenting step can also be performed on the wire rod or on a steel wire drawn
from the
wire rod via known patenting technologies using either appropriate molten
metals baths
(such as Pb) or alternatives like fluidized bed, molten salts and aqueous
polymers. Prior
to wire drawing, a pickling and wire coating step can be performed.
[53] Optionally, a phosphate coating can be applied on the steel wire
before the wire drawing
process. The step of helically coiling the steel wire into the helical spring
can then
performed with the steel wire comprising the phosphate coating at its surface.
Such
embodiment provides a better helical compression spring because the phosphate
coating
facilitates the wire drawing and spring coiling operation.
[54] Preferably, after the patenting operation; and before drawing or
between drawing steps, a
metallic coating is applied on the steel wire via hot dip. The metallic
coating comprises at
least 84% by mass of zinc; and optionally aluminum.
[55] Preferably, the method of making the helical compression spring
comprises the step of
thermally stress relieving the helical compression spring after coiling it.
More preferably,
the thermal stress relieving heat treatment step is performed at a temperature
below
450 C on the helical compression spring after its formation. More preferably,
the stress
relieving heat treatment step is performed at a temperature below 300 C, more
preferably
below 250 C.
[56] Optionally, other process steps can be applied to the helical
compression spring after
stress relieving, e.g. hot setting or multiple cold setting. With hot setting
is meant that the
spring is kept at an elevated temperature in compressed state during some
time. With
cold setting is meant that the spring is compressed for a number of cycles at
room
temperature. Such setting operations enable the spring to achieve more strict
limited
spring relaxation requirements.
[57] The third aspect of the invention is an actuator for opening and
closing a door or a
tailgate of a car. The actuator comprises a helical compression spring as in
any
embodiment of the first aspect of the invention, for opening a door or the
tailgate of a car
when compressive forces of the helical compression spring are released; and a
motor.
The motor is provided for compressing the helical compression spring in order
to close
the door or the tailgate of the car. Preferably, the actuator comprises two
connectors, one
for connecting the actuator to the door or to the tailgate; and the other one
for connecting
the actuator to the body of the car.
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[58] In preferred actuators, the helically coiled steel wire comprises a
metallic coating layer
comprising at least 84% by weight of zinc. More preferably, the metallic
coating layer
provides the surface of the helical compression spring. Such embodiments have
the
benefit that noise in the actuator is prevented when driving the car. It is
common practice
5 in
prior art actuators for opening and closing a door or a tailgate of a car, to
apply a flock
layer on the helical compression spring: a layer of short textile fibers (e.g.
polyamide
fibers) are bonded by means of an adhesive onto the helical compression
spring, after
coiling of the spring; this way, a velvet layer is created that acts as noise
dampening on
the tightly compressed spring in the actuator. The use of the metallic coating
layer has
10 shown
to eliminate the need of applying a flock layer onto the helical compression
spring.
Brief Description of Figures in the Drawings
[59] Figure 1 shows an SUV comprising an actuator for opening and
closing its tailgate.
Figure 2 shows an actuator for opening and closing a tailgate of a car.
Figure 3 shows a helical compression spring as in the invention.
Mode(s) for Carrying Out the Invention
C (wt%) Mn (wt%) Si (wt%) Cr (wt%) V (wt%) Al
(wt%)
Alloy min max min max min max min max min max min max
A
0.84 0.88 0.60 0.70 0.15 0.35 0.20 0.30 0.04 0.09 0.02 0.06
B
0.84 0.88 0.60 0.70 0.60 0.80 0.20 0.30 0.04 0.09 0.02 0.06
C
0.80 0.84 0.70 0.85 0.15 0.35 0.20 0.30 0.05 0.08 0.02 0.06
D 0.90 0.95 0.25 0.45 0.15 0.30 0.15 0.30
0.01
E 0.90 0.95 0.25 0.45 1.10 1.30 0.15 0.30
0.01
G 0.90 0.95 0.30 0.60 1.10 1.30 0.20 0.40
H 0.90 0.95 0.30 0.60 1.10 1.30 0.20 0.40
0.04
I 0.88 0.94 0.35 0.55 1.10 1.30 0.20 0.30
J 0.90 0.94 0.35 0.55 1.20 1.40 0.20 0.40
K 0.85 0.90 0.60 0.70 0.15 0.35 0.20 0.30 0.04 0.08 0.02 0.06
Table I: Examples of steel alloys that can be used in the invention.
[60] Figure 1 shows an SUV 12 comprising a tailgate 14 and an actuator
16 for opening and
closing the tailgate. The actuator 16 (figure 2 shows the actuator 16 for
opening and
closing the tailgate of a car) comprises a helical compression spring 18 and a
motor 20.
The actuator comprises two connectors 22, 23, one for connecting the actuator
to the
door or to the tailgate; and the other one for connecting the actuator to the
body of the
car. The helical compression spring is provided for opening the tailgate when
compressive forces of the helical compression spring are released. The motor
is provided
for compressing the helical compression spring in order to close the tailgate.
An example
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of a helical compression spring that can be used is shown in figure 3, such
spring has a
length L and a pitch p.
[61] The helical compression spring comprises a helically coiled steel
wire. The diameter d (in
mm) of the helically coiled coated steel wire is between 2 and 5 mm.
[62] Table I provides specific examples of steel alloys (with minimum and
maximum wt% of
the elements in the steel alloy) that can be used for the steel core in the
invention. The
microstructure of the steel wire in the helically coiled steel wire is drawn
lamellar pearlite.
[63] A specific example of such helical compression spring has been coiled
with a steel wire
having a drawn pearlitic microstructure and 3.4 mm diameter. The helical
compression
spring has a length L 0.8 m in unloaded condition. The spring index of the
exemplary
helical spring is 6.5. The pitch p of the spring is 15.2 mm. The outer
diameter of the
helical compression spring is 26.8 mm. However not essential for the
invention, the steel
wire was provided with a metallic coating layer comprising zinc and aluminum.
[64] In order to manufacture the steel wire used for coiling the helical
compression spring, a
steel wire rod of 10 mm diameter was used.
[65] The steel wire rod was out of a steel alloy consisting out of 0.86 wt%
C, 0.63 wt% Mn, 0.2
wt% Si, 0.22 wt% Cr, 0.06 wt% V; 0.04 wt% Al; unavoidable impurities and the
balance
being iron. This is an alloy of composition "A" of table I. The carbon
equivalent is: 0.86 +
(0.63/6) + (0.2/5) + (0.22/5) + (0.06/5) = 1.169.
[66] The 10 mm diameter steel wire rod has been patented to provide it with
a pearlitic
microstructure; and ¨ although not essential for the invention - has then been
provided
with a metallic coating via hot dip. The hot dip process used was a double dip
process in
which the steel wire was first dipped in a bath of molten zinc; followed by
dipping the steel
wire in a bath comprising 10 `)/0 by weight of aluminium and the remainder
being zinc. The
metallic coating layer of the hot dipped steel wire consisted of 10 wt%
aluminum and the
balance being zinc.
[67] The patented ¨ and hot dipped - wire rod of 10 mm diameter has been
drawn to a steel
wire of 3.4 mm diameter; this means that a drawing reduction of 88.4% has been
applied.
The resulting steel wire has a drawn pearlitic microstructure. The tensile
strength Rm of
the steel wire is 2354 MPa; the Rp0.2 value is 1990 MPa, which is 84.5 cYo of
the Rm
value. The percentage reduction of area Z at break in tensile testing of the
steel wire is
44.1 `Yo.
[68] The metallic coating on the drawn wire of 3.4 mm was 45 g/m2.
[69] After coiling this coated steel wire into a helical compression spring
a thermal stress
relieving operation was performed, e.g. by keeping the helical compression
spring in
unloaded condition at 250 C during 30 minutes.
[70] The coated steel wire comprised an intermetallic coating layer between
the steel core and
the metallic coating layer. The intermetallic coating layer provided 45 cYo of
the combined
thickness of the intermetallic coating layer and the metallic coating layer.
The intermetallic
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coating layer comprises a Fe,Aly phase. It has been observed that the metallic
coating
layer comprised a globularized aluminum rich phase.
[71] Samples of the steel wire used for making the helical spring have been
subject to a
thermal treatment in an oven during 30 minutes at an oven temperature of 250
C. After
this thermal treatment, tensile testing has been performed on the steel wire
sample: the
tensile strength Rm is 2426 MPa; the Rp0.2 value is 2366 MPa, which is 97.5
`)/0 of the
tensile strength Rm; and the percentage reduction of area Z at break was 42%.
[72] Analysis of the steel wire of the helical compression spring has shown
that the steel has a
drawn pearlite microstructure, with more than 97 cYo by volume of drawn
pearlite and
about 1 cYo by volume of bainite.
[73] The helical compression spring was used in an actuator for a tailgate
opening and closing
actuator of a car. The metallic coating of the coated steel wire provided the
surface of the
helical compression spring.
